Powered dental scalers commercially available on the market may generally be classified into two categories; electromechanical ultrasonic scalers, and air-driven subsonic scalers. The scalers of the former category may be grouped into those having magnetorestrictive oscillators and those having piezoelectric oscillators. Japanese Unexamined Patent Publication No. 59-25738, published Feb. 9, 1984; Japanese Unexamined Patent Publication No. 60-55941, published Apr. 1, 1985; and, Japanese Unexamined Utility Model Publication No. 53-71992 published Jun. 16, 1978, disclose examples of the prior art ultrasonic dental scalers with magnetorestrictive oscillators. The magnetorestrictive oscillator includes a coil winding and a magnetorestrictive transducer disposed within the winding. A high-frequency alternating current having a frequency in the range of about 20 to 40 kHz is supplied from a control unit to the coil winding to generate high-frequency alternating magnetic fields which induce ultrasonic acoustic vibrations in the magnetorestrictive transducer. The ultrasonic vibrations are transmitted through an acoustic coupling to a scaler tip to cause the tip to vibrate at an ultrasonic frequency. The piezoelectric oscillator, on the other hand, includes a piezoelectric transducer which is oscillated by a similar high-frequency alternating current applied thereon from a control unit. In both types of oscillators, the nature of the vibrations as generated in the transducer is acoustic, so that the sound waves or elastic waves generated therein are propagated axially through the transducer and through the acoustic coupling to the scaler tip to cause the scaler tip to vibrate at an ultrasonic frequency. Thus, the primary advantage of such electromechanical ultrasonic dental scalers, including magnetorestrictive or piezoelectric oscillator types, is that they are operable without producing an audible keen noise which would normally be encountered when the dentist is operating a turbine driven instrument, and which generally would have a considerably adverse affect on the feeling of a patient. Nevertheless, the electromechanical scalers have a disadvantage in that the control unit must be installed on or in the neighbourhood of the dental unit. The provision for such control unit also entails extra cost for the dentists.
Air-driven dental scalers are generally designed to be driven by a source of compressed air provided in an existing dental unit which is standard equipment in almost all dental clinics and, thus, have the advantage of being interchangeably usable in place of turbine-driven dental instruments by a simple connection to a flexible hose extending from the dental unit. Thus, there is no need for a separate control unit. Air-driven dental scalers includes various types of vibrators or oscillators. For example, U.S. Pat. No. Re. 29,687 reissued Jul. 4, 1978, describes a dental scaler having a central shaft resiliently supported adjacent both ends thereof by a casing. A sleeve-like rotor is rotatably mounted around the shaft at the center thereof and is adapted to be rotated by an offset jet of air to cause subsonic vibration of the shaft. The dental scaler disclosed in Japanese Unexamined Patent Publication No. 56-83341 published Jul. 7, 1981, employs a vibrator having a similar sleeve-like rotor rotatable around a shaft. Japanese Unexamined Patent Publication No. 56-166842 illustrates an air-driven dental scaler with another type of vibrator having an air turbine, the rotation of which is transformed by an eccentric to an oscillatory movement of a shaft coupled to the scaler tip.
Throughout these types of air-driven dental scalers, the mode of vibration of the vibrator shaft to which the scaler tip is mounted is entirely different from the mode of vibration encountered in the electromechanical dental scalers, wherein elastic waves oscillating at ultrasonic frequencies are generated in the magnetorestrictive or piezoelectric transducer and are axially transmitted to the scaler tip. For example, in the dental scaler described in U.S. Pat. No. Re. 29,687, the vibrator shaft carrying the scaler tip is resiliently supported adjacent the ends thereof by the casing of the scaler, and an external vibratory force is imparted from the rotating sleeve-like rotor at about the center of the shaft, so that the vibrator shaft undergoes flexural forced vibration, as opposed to elastic vibration, with the nodes of flexural vibration located at the points at which the shaft is supported by the casing. Obviously, the flexural nature of the mode of vibration has made it necessary to design the vibrator shaft to be long enough to cause the scaler tip to vibrate with a sufficient amplitude of vibration required for scaling. This has been a bar to increasing the frequency of vibration of the air-driven subsonic dental scalers to near to the ultrasonic range, the lower boundary of which is generally from 15 to 20 kHz. In fact, an annoying keen noise having a frequency of about 6,000 Hz has often been encountered in the prior art air-driven dental scalers referred to above. It appears to be taken for granted that the generation of audible sound or noise is inherent in the air-driven dental scalers, and that in the field of air-driven scalers, it is almost impossible to avoid annoying noise due to subsonic vibration. Obviously, no attempt has been hitherto made to improve the above described subsonic scalers in such a manner that the frequency thereof is increased to near to the ultrasonic frequency range.
Another type of air-driven dental scaler is described in U.S. Pat. No. 4,453,919 issued to Takeshita on Jun. 12, 1984 and assigned to the assignee of the present invention. This dental scaler comprises a unique air vibrator having a vibrator body defining a disk-like chamber in which a disk-like rotor is received. Compressed air from a source in the dental unit is injected into the chamber through tangential inlet ports or nozzles to generate in the chamber a swirling air stream that causes the rotor to rotate. Although it is not entirely clear how the vibration is generated in response to rotation of the rotor, the mechanism of the vibration is described by analogy to a wobbling coin spinning on a table surface, and striking that surface. In the preferred embodiment, Takeshita provides an elongated shaft connected at an end to the vibrator body and at the other end to the scaler tip to transmit the vibration generated in the vibrator body to the scaler tip. The shaft is resiliently supported by the casing of the scaler at a point where the node of flexural vibration is located. The vibrator, together with the vibration transmission shaft and the scaler tip, make up a vibration system which undergoes subsonic vibration in response to vibration generated in the vibrator.
Although the air-driven dental scaler of Takeshita is very effective in removing calculus from teeth and enjoys the advantage of being readily connected to the hose from the dental unit interchangeably with the turbine handpieces, the problem which must be overcome in the design of the Takeshita scaler is that it still generates an audible keen sound or noise which is commonly encountered in the various types of air-driven dental scalers.